The present invention relates to methods of producing germanium tetrafluoride. More particularly, but not by way limitation, the present invention relates to methods for producing ultra high purity germanium tetrafluoride suitable for use in, for example, semiconductor applications.
Several processes are known in the art for producing technical grade germanium tetrafluoride. One method involves the fluorination of germanium with undiluted fluorine or nitrogen trifluoride gas wherein the germanium is placed in a batch reaction chamber and the chamber is filled with the undiluted gas. The batch reactor is then allowed to set at a pressure of about 3 atmospheres and a temperature of from 250-500xc2x0 C. Another production method involves the decomposition of barium fluorogermanate in a quartz tube at 750-1000xc2x0 C. Another method involves the decomposition of hexafluorogermanic acid at 400-600xc2x0 C. Another method utilizes halogen exchange between GeCl4 and SbF3 to yield a mixture comprising germanium tetrafluoride, GeF3Cl, GeF2Cl2 and GeFCl3. Still another method involves the reaction of sulfuric acid with a mixture of germanium oxide and one or more metal fluorides. Yet another method involves the reaction of germanium with uranium tetrafluoride.
Unfortunately, each of these prior techniques produces a substantial amount of volatile and/or nonvolatile impurities that are difficult and costly to separate from the desired reaction product. Examples of such impurities include: silicon tetrafluoride, sulfur hexafluoride, carbon tetrafluoride, NOx, potassium fluorochlorides, hydrogen fluoride, germanium difluoride, transition metal fluorides, and elemental fluorine.
Thus, a need presently exists for a less complex and more cost effective method for producing high purity germanium tetrafluoride. Such method will also preferably provide a high product yield. A need particularly exists for a much less complicated and more cost effective approach for producing ultra high purity germanium tetrafluoride suitable for use in photovoltaic cells, in nanocrystals, as an ion implantation dopant, or in other semiconductor applications.
The present invention provides a method of producing high purity germanium tetrafluoride which satisfies the needs and alleviates the problems discussed above. The inventive method comprises the step of flowing a gas mixture comprising fluorine gas and an inert gas through a reactor having germanium therein. The fluorine gas contained in the gas mixture reacts with the germanium to produce germanium tetrafluoride which is included in the reactor product gas stream. The inventive method preferably also includes the step, prior to flowing the gas mixture through the reactor, of purifying the fluorine gas by cryogenically chilling to a temperature of less than xe2x88x92150xc2x0 C. and contacting the chilled fluorine gas with sodium fluoride.
The inventive method also preferably comprises the step of conducting the reactor product stream through at least one product trap maintained at a temperature effective for condensing at least a portion of the germanium tetrafluoride from the reactor product stream. The condensed germanium tetrafluoride is preferably retained in the product trap.
In addition, the inventive method preferably includes the step of purifying the condensed germanium tetrafluoride to remove volatile impurities. The volatile impurities are preferably removed by subliming the germanium tetrafluoride in a product container through one or more xe2x80x9cfreeze/thawxe2x80x9d cycles and then evacuating the head gas from the product container to remove vaporized impurities therefrom. The steps of subliming and evacuating are preferably repeated until essentially all of the volatile impurities are removed.
In yet another aspect, the inventive method preferably further includes the step of removing nonvolatile impurities from the germanium tetrafluoride product by (a) sublimating the germanium tetrafluoride, in a container, to a vapor state and (b) transferring the sublimated germanium tetrafluoride out of the container such that the nonvolatile impurities are separated from the germanium tetrafluoride vapor.
Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawing and upon reading the following detailed description of the preferred embodiments.